Sample preparation and analysis presents many logistical problems. Conventionally, many medical samples (such as blood, saliva, urine and swab eluate) are provided to a doctor, for example a general practitioner doctor (GP) or a principle care physician (PCP), in a local surgery without the equipment necessary to analyse the sample. Hence, the sample must be sent to a laboratory where the sample is analysed. The test results must then be collated and returned to the GP to analyse the results and make a diagnosis. This approach is inadequate. Firstly, there is a significant risk that a sample is lost in transit or mismatched with the wrong patient. Moreover, whilst recent developments in technology have reduced the overall time taken to conduct the test, the delay involved in sending the sample to a laboratory is unsatisfactory.
Nevertheless, analytical systems of the kind found in laboratories are complex and it is often difficult to provide sufficient amounts of pure targets from source samples to reliably perform downstream analytical assays. This typically prohibits local GP surgeries from being able to carry out such tests on site.
However, in recent years efforts have been made to reduce the scale of the analytical systems to make tests faster and simpler to run, and require smaller quantities of sample. For instance, “laboratory on a chip” (LOC) devices (a subset of microfluidic devices) integrate almost all medical tests or diagnostic operations performed in a hospital on a single microfluidic chip. The channels forming such microfluidics devices handle small fluid volumes and are connected together so as to achieve a desired function such as mixing of a sample, moving the sample through the device, reacting the sample with different reagents, and so on. These chips may be inserted into machines to control the performance of a test and measure the results.
However, it has been found that handling a sample in a microfluidics device can be very difficult. In particular, it is difficult to interface to the small channels and other features that are required to move the sample from one site to another to perform different actions on the sample. There is also a limit to the complexity of a LOC device which operates purely using capillary action. Furthermore, owing to the small sample sizes of LOC's, the devices have reduced sensitivity and the probability of a target being present in the sample is thus reduced.
An alternative approach is to use a fluidic cartridge. The scale of the components of a fluidic cartridge is larger than for a microfluidic device, and so it becomes possible to move a sample through various different sites to perform different actions on it. This makes it possible to perform more complex tests than may be conducted using typical LOC devices, whilst still providing an analytical system of potential use in a local GP surgery.
Fluidic cartridges are generally inserted into a cartridge reader configured to initiate and control at least some of the steps of a test to be carried out. For example, cartridge reader may initiate a test by detecting the presence of a cartridge and moving a sample through the various channels in the cartridge. The reader may initiate the introduction of required reagents into the cartridge, and control variables such as sample temperature throughout the duration of the test. Finally, the reader may be configured to read and display a result to the user, once the required test has been carried out.
Increasingly, scientific assays useful in medical diagnostics have involved biochemical procedures, such as the polymerase chain reaction (“PCR”). The PCR assay has provided a particularly sensitive method of assaying for the presence of defined segments of nucleic acids. It is therefore desirable to perform a PCR assay on a fluidic cartridge, and to provide a cartridge reader, suitable for use in a local surgery of doctors' office capable of carrying out and/or controlling a PCR assay. The use of PCR requires rapid and reliable thermal control on the cartridge,
Reducing PCR to the microchip level is important for portable detection technologies and high throughput analytical systems. The method can be used to assay body fluids for the presence of nucleic acid specific for particular pathogens, such as the Chlamydia trachomatis bacterium, HIV or any other pathogenic microbe.
The introduction of commercially available automated DNA amplification assays has allowed more laboratories to introduce these technologies for routine testing of specimens. However, there is a need to improve the cartridges and cartridge readers used for this purpose.
Electrochemical signalling may be used to indicate the presence of genetic or immuno-histochemistry targets in a sample. The sample is processed to form an electrolyte which, in practice, may be held in a cell comprising a set of detection electrodes. Upon application of a potential difference across electrodes in the cell, some compounds in an electrolyte will have a natural tendency to migrate to the electrodes and swap electrons, resulting in a tiny current. All combinations of soluble compounds have some electrochemical activity, and the rate at which this activity occurs enables measurement of the quantity of those compounds. Thus, the presence of different compounds in the sample may be measured by searching for characteristic features of their redox electrochemistry. In particular, the sample may be processed to include labels: selected compounds that are present if and only if the sample contains target molecules.
A circuit used to measure electrochemical activity is a potentiostat, which has three electrodes—a working electrode, a counter electrode and a reference electrode. A potential difference is applied across the working and counter electrodes and, as a result, a label indicating the presence of target DNA oxidizes on the working electrode and a current flows from the working electrode to the counter electrode. This current is dependent of the galvanic activity (natural reactivity between electrode and electrolyte) at both electrodes, and in order that only the effect of the working electrode electrochemistry is measured, the potential difference applied is corrected for the galvanic activity of the counter electrode by an amount determined by the reference electrode which is chosen to be a ‘standard electrode’ relatively unchanged by local chemistry effect.
The current flowing at any given thus-corrected potential difference is measured and provides the signal that is indicative of the label compounds in the sample. Conventionally, the counter electrode is excessively large so that the reaction at this electrode does not limit the current flowing as a result of the reaction of the working electrode, which is the one of interest.
However, during development of a cartridge and a cartridge reader as described above, the inventors found that the signals generated by potentiostats of conventional design were unsatisfactory, and thus had a need for an improved arrangement that generated better signals.